A thin film transistor formed over a flat plate such as a glass substrate is formed using amorphous silicon or polycrystalline silicon, as typically seen in a liquid crystal display device. Although a thin film transistor including amorphous silicon has low field effect mobility, it can be formed over a larger glass substrate. In contrast, although a thin film transistor including polycrystalline silicon has high field effect mobility, it needs a crystallization process such as laser annealing and the threshold voltage greatly varies, so that such a transistor is not always suitable for a larger glass substrate.
In contrast, attention has been drawn to a technique by which a thin film transistor is formed using an oxide semiconductor and is applied to an electronic device or an optical device has attracted attention. For example, Reference 1 discloses a technique by which a thin film transistor is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor for an oxide semiconductor film and is used as a switching element or the like in a liquid crystal display device.